It has been known that particles heavy enough to be displaced in such a suspension by gravitational or centrifugal means can be so separated therefrom by well known apparatus, such as settling chambers or dry cyclones. It is also known that somewhat smaller particles can be made heavy enough by wetting, making it possible to separate them in other known apparatus, such as wet cyclones or scrubbers.
However, many constituents of particulate aerosols (for purposes of this application aerosols may be defined as suspensions, in a gaseous medium, comprising particulate matter not heavy enough to be separable by gravitational or centrifugal means) are so small that they are hard to wet either by sprays directed toward them or by bodies of liquid through which the aerosols are bubbled or against which they are impinged. The relatively large surface area exposed by sub-micron particles surrounds them with gaseous envelopes which tend to prevent direct contact between the wetting liquid and the unwet particles. They can however be wet by super-saturating the gaseous medium with the vapor of a liquid which is thus condensed directly from the vapor phase upon the surfaces of the suspended particles. This happens naturally when clouds are formed upon nuclei suspended in rising air currents.
Scrubbers designed in the past to make use of this principle have been handicapped by their tendency to wet only the largest of the particles present because the vapor pressure of a mist droplet at a particular temperature varies inversely with its diameter. Therefore, when the aerosol becomes supersaturated, the first droplet formed by condensation upon the surface of the largest particle will grow cumulatively larger as its increasing size reduces its vapor pressure farther below the particle pressure of the uncondensed vapor. Thus the vapor tends to condense preferentially upon a growing droplet of liquid rather than upon adjacent smaller dry particles.
For the same reason, when a vapor-bearing aerosol is cooled through contact either with a colder surface or a spray of colder liquid, not only will condensation occur preferentially upon the coolant, but existing smaller mist droplets will tend to dry out through vapor transfer to the larger colder surfaces and the smallest dry particles will escape with the scrubbed gas output. Therefore, scrubbers designed to wet particulate aerosols by the injection of steam have been faced either with the loss of significant quantities of heat and water vapor or the escape of the smaller particles.